Many thanks for the answers so far - I'm more than interested to note that the majority are suggesting the same average figure of 4%.
Back to the original question, however, for the other two or three replies - I asked (in bold italics) when does it make a significant difference.
Minimum 737? Really? Not a Lear Jet? Not an ATP, Shorts 360, Citation? Gulfstream V?
I already said I knew it would burn more even in a "spam can" but when would it make a significant difference.
What the flight time v. miles has to do with, I've no idea.
Just because it's somebody elses fuel in a rental doesn't mean I should consider flying it any different, or less professionaly (does it?)....and no, I don't fly at a fixed power setting. I fly at the power setting required to achieve the performance I desire.